Electrodes for electrolytic purposes



United States Patent 3,331,709 ELECTRODES FOR ELECTROLYTIC PURPOSES John Hill, Uxbridge, Middlesex, and Norman Maynard Hopltin, London, England, assignors to Johnson, Matthey & Company Limited, London, England, a British company No Drawing. Filed Apr. 7, 1965, Ser. No. 446,391 Claims priority, application Great Britain, Apr. 9, 1964,

12 Claims. (Cl. 1482) ABSTRACT OF THE DISCLOSURE A method of producing soluble silver anodes from high purity silver by removing oxygen from. the silver and preventing re-oxidation, thereby making electrodes having a reproducibly consistent performance with an absence of coarse or fine powdered silver falling away during use.

This invention relates to improvements in and relating to electrodes for electrolytic purposes and is more particularly concerned with the production of soluble silver anodes for use in the electrodeposition of silver.

Electrolytic anodes, as at present used, generally consist of an annealed sheet or strip of silver having a predetermined hardness below 40 VPN and of a particular shape designed to ensure as uniform as possible a dissolution of the anode throughout the whole periphery of the sheet or strip. It has also been the custom to fabricate the anodes from sheet or strip having crystals of a uni form size and to use silver of a relatively high state of purity.

Silver anodes, fabricated in the manner referred to above, have not proved entirely satisfactory in practice, as consistency in performance of the anodes cannot be ensured, one anode functioning satisfactorily, whilst another, although made in the same manner from the same material, behaving entirely unsatisfactorily due to what is known as shedding. Shedding, which term is used in the art to mean the falling away from the anode of coarse or fine powdered silver, detrimentally affects the performance of a plating operation and anodes, which are subject to this characteristic in use, cannot be regarded as eflicient. In the first place, relatively coarse particles of silver, which fall away from the anode, are not usable for plating purposes, as, owing to their loss of electrical contact, they are not dissolved in the electrolyte and take no part in the plating operation. Moreover, if the shedding is in the form of fine particles, the latter will circulate in the electrolyte, and, particularly if the electrolyte is stirred, will eventually settle on the article being plated and result in a roughened deposit of silver thereon.

In an endeavour to ascertain the reason for this inconsistent behaviour of silver anodes as heretofore made, the applicants have carried out investigations into the manufacturing processes involved and have quite unexpectedly found that the difiiculties hitherto associated with the production of silver anodes can be readily overcome and that anodes, having a predetermined consistent performance, can be obtained.

During the course of their investigation, the applicants found that the factors, hitherto regarded as of prime importance, namely, crystal size and hardness of material and the shape of the finished anode, did not in fact have, in any way, a decisive effect on the performance of an anode, but that certain other factors, hitherto disregarded, were of vital significance. The applicants have, in fact, surprisingly found that satisfactory anodes, from which consistent performances can be expected, can be obtained provided, during manufacture, the oxygen content of the silver and the homogeneity of the silver crystals are carefully controlled.

It has been found that, if oxygen is present in the material from which an anode is made, the oxygen will, in the use of the anode, tend to collect at the crystal grain boundaries in the form of oxides of any base metal impurities present. These oxides will dissolve more rapidly than the silver crystals themselves and will tend to undermine the latter and result in coarse shedding, or fall-out from the anode of coarse powder particles of silver which, as pointed out above, remain undissolved in the electrolyte.

It has further been found that, whilst reduction of the oxygen content is of major importance in reducing shedding, the absence of oxygen will cause any impurities in the anode material to be distributed through the crystals as metals instead of accumulating at the grain boundaries as oxides. This distribution of impurity metals will, however, not be uniform with the result that difierent portions of the crystal will dissolve at different rates, which will again result in the phenomenon of shedding, albeit in the form of fine particles, which, as previously mentioned, are liable to cause roughening of the deposit.

The relative coarseness or fineness of the particles shed is, in no way, determinative of the total weight of the fall-out of either type, which may occur in practice, as a greater Weight of fine particle fall-out than of coarse particle fall-out is a not infrequent occurrence.

The object of this invention is to overcome the disadvantages hitherto associated with the manufacture of silver electrolytic anodes and to provide a novel method of manufacture of anodes which will ensure consistently reproducible performance from anode to anode.

With the above and other objects in view, the invention provides a method of making a silver electrode intended for electrolytic purposes which includes the steps of melting silver of about 99.98% purity, treating the molten silver with a reducing agent so as to reduce any oxygen present in the initial silver, forming the so treated silver into a billet of desired electrode shape and heat treating the shaped electrode at a temperature above 400 C. for a period of not less than 2 hours to effect homogenisation of silver crystals within the electrode structure.

The invention further provides a method of treating a silver electrode to reduce to a minimum the tendency of said electrode to shed in use which comprises the steps of reducing the oxygen content of said electrode so as to provide an oxygen-free, or substantially ogygen-free electrode and then heat-treating the said electrode to effect homogenisation of the crystals thereof.

In carrying out the invention in practice, it will be found preferable to make use of silver of as high a purity as possible, for example, 99.98% or better, apart from copper, the content of which may amount to as much as 0.1% by weight, as, otherwise, the impurities present will, when the anode is used in an electroplating bath, contaminate the electrolyte.

The silver should preferably contain less than 0.0015% by weight of oxygen.

Electrodes embodying the invention are preferably made by a continuous casting procedure, although other methods, such as extrusion, may, if desired, 'be employed. It will, however, be found that the casting method has the advantages of facilitating the control of the oxygen content as required and of producing a structure of interlocking crystals which tends to assist in the avoidance of shedding.

The crystal homogenising step consists in an annealing treatment, the temperature and duration of which do not appear to be critical and may be varied over relatively wide ranges. In carrying out the invention in practice,

the homogenising step may advantageously comprise annealing the silver at a temperature within the range of from about 400 C. up to 950 Cpfor a period which may vary from about 2 hours up to about 16 hours. The annealed silver should then be cooled to below 200 C., before exposure to air, and then quenched in water.

In order that the invention may be fully understood, one embodiment thereof will now be described by way of example, it being clearly understood that the invention is in no way limited 'thereto or thereby.

Silver of about 99.98% purity is melted under a carbon cover by placing small pieces of charcoal on the surface of the melt and the molten silver is stirred with a graphite rod, so as to remove as far as possible any oxygen, which may have been present in the original silver. Thermolten silver at a temperature of :1090 C., is poured into a con- ;tinuous casting machine and thesolidified metal, on issuing from the mould, is rapidly quenched by a water-spray to reduce, to an absolute minimum, the risk of oxygen pick-up from the atmosphere.

. -The actual temperature of pouring may Vary from the above figure, dependent on the particular typeof casting machine used to carry out the casting operation.

- When the casting has been elfected, the silver billet,

I which now should have an oxygen content of not more than 0.0015% by weight, is cut to required sizes for electrode-forining purposes and the pieces, so obtained, are

inserted into a stainless steel box, to which an inert atmosphere may be admitted, and which is then placed in an annealing furnace. A protective atmosphere of pure dry nitrogen is used, but if desired, carbon dioxide or argon may be used instead.

The silver is annealed to effect homogenisation of the crystals at a temperature of 750 C. for a period of 2 hours and is then quenched in water or allowed to cool.

Electrolytic silver anodes, in accordance with this invention, will be found to be highly satisfactory in use under normal plating conditions and to be free, in use, from shedding or to exhibit shedding characteristics to a minimumextent; V For example, silver anodes treated in accordance with the teaching of this invention, when testedunder conditions normally liable to cause severe shedding and which are, therefore, not employed in normal plating practice, namely, in use in an electrolyte of the following composition:

. G./l. Silver as silver potassium cyanide 60 Free potassium cyanide 60 Potassium carbonate 24 at "a temperature of 40 C. and a current density of 2 amps/ sq. ft. exhibited, after use, a shedding of less than iAnodes, which had not been treated in accordance with this invention, when subjected to the same'operating conditions as above, were found to have a very variable per- .formance and, in some cases, shed to an extent of more than 1%. 7

It will, thus, be readily appreciated that theimproved silver anodes of the invention will have a greatly improved operating life and the user may select any anode from any given batch of such anodes with the knowledge that it will offer the same satisfactory performance as will any of the others of the batch.

It is to be understood that the invention is intended to include within its scope silver electrodes, such as electrohas been described, it is to be understood that modifications may be made thereto. without departing from the scope of the invention. For example, the electrode need not be made by a continuous casting process, but an extrusion process may be used instead.

What we claim is:

1. A method of making a Silver electrode intended for electrolytic purposes which includes the steps of melting silver of about 99.98% purity, treating the molten silver with a reducing agent so as to reduce any oxygen present in the initial silver, forming the so treated silver into a billet of desired electrode shape and heat treating the shaped electrode at a temperature above 400 C. for a period of not less than 2 hours to effect homogenisation of silver crystals within the electrode structure.

2. A method according to claim 1 wherein the'so treated silver is cooled prior to being formed into a billet of desired electrode shape. a r

3. A method according to claim 1 wherein the so treated silver is continually cast into billet material.

4. A .method according to claim 1 wherein the so treated silver is cast and the casting mechanically worked to form a billet of the desired electrode shape.

, 5. A method according to claim 1 wherein the reducing agent is a carboniferous material. 6. A method of making a silver electrode which comprises the steps of melting silver of about 99.98% purity in the absence of air and in the presence of carboniferous material, thereby removing oxygen, casting the molten silver and quenchingthe molded metal with a water-spray,

thereby reducing the risk of oxygen pick-up from the atabout 2 hours up to about 16 hours and then quenching V the electrode. p

9. A method according to claim 6 in which the electrode is annealed at a temperature of 750 C. of 2 hours.

10. A method according to claim 1 in which the electrode is heated to from 400 to 950 C. for a period of from about 2 hours up to "about 16 hours and is then 7 quenched.

11. A method according to claim 7 in which the an-. nealing treatment consists in heating the electrode from 400 to 950 C. for a period of from about 2 hours up to about 16 hours and is then quenched.

12. A method according to claim 1 in which the electrode is heat-treated by annealing at a temperature of 750 C. for a period of 2 hours.

0 References Cited UNITED STATES PATENTS 2,802,782 8/1957 Bayes et a1. 204'292 FOREIGN PATENTS 769,193 2/1957 Great Britain.

HYLAND BTZOT, Primary Examiner. DAVID L. RECK, Examiner. V H. F. SAITO, Assistant Examiner.

for a period A 

1. A METHOD OF MAKING A SILVER ELECTRODE INTENDED FOR ELECTROLYTIC PURPOSES WHICH INCLUDES THE STEPS OF MELTING SILVER OF ABOUT 99.98% PURITY, TREATING THE MOLTEN SILVER WITH A REDUCING AGENT SO AS TO REDUCE ANY OXYGEN PRESENT IN THE INITIAL SILVER, FORMING THE SO TREATED SILVER INTO A BILLET OF DESIRED ELECTRODE SHAPE AND HEAT TREATING THE SHAPED ELECTRODE AT A TEMPERATURE ABOVE 400*C. FOR A PERIOD OF NOT LESS THAN 2 HOURS TO EFFECT HOMOGENISATION OF SILVER CRYSTALS WITHIN THE ELECTRODE STRUCTURE. 